High-dimensional precision is required on the end portion of a metal pipe or tube supplied for service as a line pipe or as oil country tubular goods. In the supply of the service, a line pipe is usually welded to its adjacent line pipe. If the inside diameter of the end portion of a line pipe does not precisely meet with that of the adjacent line pipe, it leads to trouble with welding which causes defects of the welded portion. Ordinary oil country tubular goods are subjected to threading operation on the end portions in order to connect them to their adjacent oil country tubular goods. If the precision of the inside diameter of the parent oil country tubular goods are poor, the threading operation cannot be completed properly.
In order to improve the precision of the inside diameter of the end portions of a metal pipe or tube, the end portions are expanded.
The equipment for the expanding operation includes a chuck 2, a plug 3, and a cylinder 4 as shown in FIGS. 1A, 1B, and 1C. Starting from the head to the tail of the plug 3, the geometry of the plug 3 includes the taper portion 31 which smoothly connects to the parallel portion 32. The diameters at both ends of the taper portion 31 are D10 on the head end and D11 on the tail end, with D11 being larger than D10. The taper angle R1 of the taper portion 31 is constant. The diameter of the parallel portion 32 is uniform throughout the longitudinal direction and is given as D11.
Prior to the expanding operation of an end portion of a metal pipe (or a metal tube) 1, the metal pipe 1 is tightly fixed to the equipment using the chuck 2. In fixing the metal pipe 1, its center axis is arranged so that it precisely meets with the center axis of the plug 3 as shown in FIG. 1A. Then the plug 3 is pushed into the metal pipe 1 to the prescribed distance in the axial direction from the end point as shown in FIG. 1B. The plug 3 is pushed into the metal pipe 1 by using the cylinder 4. The end portion of metal pipe 1 is expanded accordingly.
After the plug 3 travels the prescribed distance from the end point of the metal pipe 1, the plug 3 is pulled back in the direction opposite to the direction that it was pushed in as shown in FIG. 1C. Through this procedure the end portion of the metal pipe 1 is finished so that the precision of the inside diameter of the end portion precisely meets the prescribed value. Improvement of the dimensional precision of the inside diameter of the end portion of the metal pipe 1 is obtained accordingly.
A problem, however, is that there is a difference in the inside diameter in the circumferential direction on the expanded end portion of the metal pipe, and the inside geometry of the cross section is not a perfect circle. There is also a difference in the inside diameter in the axial direction.